For the future provision of mobile telecommunications services, it is envisaged that a number of air interface technologies will be deployed with overlapping coverage areas and that such a mixed deployment of system technologies will be required in an attempt to address specific services and environments.
In the US, both Time Division Multiple Access (TDMA) schemes (such as U.S. Digital Cellular USDC) and Code Division Multiple Access (CDMA) schemes (such as a variant of the Qualcomm system) are likely to be deployed as a wide area cellular service for the Personal Communications System (PCS). WACS (Wireless Access Communications Systems, as defined by Bellcore) is another TDMA system and is likely to be deployed in micro-cellular and pico-cellular environments. In Europe, CDMA third generation systems (based on the RACE CODIT research project) may co-exist with second generation TDMA systems such as Global System for Mobile Communications (GSM) and Digital European Cordless Telephony (DECT) as well as third generation TDMA systems.
With such a mixed deployment of systems, it is desirable for a handset to be both capable of multi-mode operation and of seamless handover between access technologies such as TDMA and CDMA.
FIG. 1 shows the prior art of handing-off between cells operating similar CDMA systems on different frequencies as employed by the RACE CODIT system. In this system, handoff is achieved by a mobile station (MS) and both base stations (BSs) entering a time compressed mode of operation. In this time compressed mode the mobile communications can occur at twice the bit rate for half of the time on each system.
This is possible in CDMA systems by reducing the spreading factor when in compressed mode so that the data rate increases while the chip rate remains constant. To compensate for the reduced spreading factor, which protects against co-channel interference, the power during the compressed mode is increased. In such a way a MS is able to continue its communication with the CDMA system, and simultaneously monitor and subsequently access another frequency, whilst the call is being transferred to the new system.
In order to initiate a handoff sequence, a MS monitors candidate BSs on new frequencies. FIG. 1 comprises four graphs that detail handoff between two CDMA systems: graph 10 showing the uplink transmissions between a MS and a first CDMA BS (BS1), graph 20 showing the uplink transmissions between the MS and the second CDMA BS (BS2), graph 30 showing the downlink transmissions between the first CDMA BS (BS1) and the MS and graph 40 showing the downlink transmissions between the second CDMA BS (BS2) and the MS. The term "uplink" is used to define a communication from a MS to a BS and the term "downlink" is used to define a communication from a BS to a MS.
To facilitate a monitoring of two frequencies and a handoff execution process, the MS requests from the first BS a time compressed mode of operation 12 as shown in Graph 10. Graph 30 indicates the switch to the time compressed mode of operation performed by the first BS 32. The MS monitors the activity of the second BS 34. The MS then requests from the first BS, handoff to the second BS 14. In the following time slot the MS transmits half of the time to the first BS 16 and half of the time to the second BS 22 using the time compressed mode of operation. The term "time slot" includes the switching time, guard time, ramp time and slot time.
Graphs 20 and 40 show the communications between the MS and the second BS. The link between the MS and the second BS is first established in the time compressed mode 22. After a short period the link between the MS and the second BS is then returned to a normal (non-time compressed) mode of operation 24. Graphs 10, 20, 30 and 40 also show signalling channels 18 which are used to establish and relinquish compressed mode operation. The signalling channel 18 is code division multiplexed on to the traffic channel.
A new approach is required when handoff is required between two dissimilar access technologies. Specifically, a candidate system may be a TDMA system. By definition, TDMA is time discontinuous and hence, the prior art is not sufficient to allow access to general TDMA systems since the characteristics of the compressed transmissions in the prior art are fixed at 50% duty cycle and twice the transmit power.
Thus it is desirable to have a multi-mode handset capable of handoff between a flexible time domain system and a second time domain system and a method of operation thereof.